Cellular Respiration: Harvesting Chemical Energy
Harvesting Energy

To perform their many tasks, living cells require energy from outside sources.

Energy enters most ecosystems as sunlight and leaves as heat.

Photosynthesis generates oxygen and organic molecules that the mitochondria of eukaryotes use as
fuel for cellular respiration.

Cells harvest the chemical energy stored in organic molecules and use it to regenerate ATP, the
molecule that drives most cellular work.

Respiration has three key pathways: glycolysis, the citric acid cycle, and oxidative phosphorylation.
Catabolic pathways yield energy by oxidizing organic fuels

The arrangement of atoms of organic molecules represents potential energy.

Enzymes catalyze the systematic degradation of organic molecules that are rich in energy to simpler
waste products with less energy.

Some of the released energy is used to do work; the rest is dissipated as heat.

Catabolic metabolic pathways release the energy stored in complex organic molecules.

One type of catabolic process, fermentation, leads to the partial degradation of sugars in the absence
of oxygen.

A more efficient and widespread catabolic process, cellular respiration, consumes oxygen as a
reactant to complete the breakdown of a variety of organic molecules.


Cellular respiration is similar in broad principle to the combustion of gasoline in an automobile engine
after oxygen is mixed with hydrocarbon fuel.


organic compounds + O2  CO2 + H2O + energy (ATP + heat).
Carbohydrates, fats, and proteins can all be used as the fuel, but it is most useful to consider glucose.


Food is the fuel for respiration. The exhaust is carbon dioxide and water.
The overall process is:


In eukaryotic cells, mitochondria are the site of most of the processes of cellular respiration.
C6H12O6 + 6O2  6CO2 + 6H2O + Energy (ATP + heat)
The catabolism of glucose is exergonic with a  G of −686 kcal per mole of glucose.

Some of this energy is used to produce ATP, which can perform cellular work.
Redox reactions release energy when electrons move closer to electronegative atoms.

Catabolic pathways transfer the electrons stored in food molecules, releasing energy that is used to
synthesize ATP.

Reactions that result in the transfer of one or more electrons from one reactant to another are
oxidation-reduction reactions, or redox reactions.


The loss of electrons is called oxidation.

The addition of electrons is called reduction.
The formation of table salt from sodium and chloride is a redox reaction.

Na + Cl  Na+ + Cl−
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

Here sodium is oxidized and chlorine is reduced (its charge drops from 0 to −1).
Redox reactions require both a donor and acceptor.
The “fall” of electrons during respiration is stepwise, via NAD+ and an electron transport chain.

Cellular respiration does not oxidize glucose in a single step that transfers all the hydrogen in the fuel
to oxygen at one time.

Rather, glucose and other fuels are broken down in a series of steps, each catalyzed by a specific
enzyme.

At key steps, electrons are stripped from the glucose.

In many oxidation reactions, the electron is transferred with a proton, as a hydrogen atom.

The hydrogen atoms are not transferred directly to oxygen but are passed first to a coenzyme called
NAD+ (nicotinamide adenine dinucleotide).

How does NAD+ trap electrons from glucose?

Dehydrogenase enzymes strip two hydrogen atoms from the fuel (e.g., glucose), oxidizing it.

The enzyme passes two electrons and one proton to NAD+.

NAD+ functions as the oxidizing agent in many of the redox steps during the catabolism of
glucose.

Each NADH molecule formed during respiration represents stored energy. This energy is tapped to
synthesize ATP as electrons “fall” from NADH to oxygen.

How are electrons extracted from food and stored by NADH finally transferred to oxygen?

Unlike the explosive release of heat energy that occurs when H 2 and O2 are combined (with a
spark for activation energy), cellular respiration uses an electron transport chain to break the fall
of electrons to O2 into several steps.

The electron transport chain consists of several molecules (primarily proteins) built into the inner
membrane of a mitochondrion.

Electrons released from food are shuttled by NADH to the “top” higher-energy end of the chain.

At the “bottom” lower-energy end, oxygen captures the electrons along with H+ to form water.

Electron transfer from NADH to oxygen is an exergonic reaction with a free energy change of −53
kcal/mol.

Electrons are passed to increasingly electronegative molecules in the chain until they reduce oxygen,
the most electronegative receptor.

In summary, during cellular respiration, most electrons travel the following “downhill” route: food 
NADH  electron transport chain  oxygen.
These are the stages of cellular respiration: a preview.

Respiration occurs in three metabolic stages: glycolysis, the citric acid cycle, and the electron transport
chain and oxidative phosphorylation.

Glycolysis occurs in the cytoplasm.


It begins catabolism by breaking glucose into two molecules of pyruvate.
The citric acid cycle occurs in the mitochondrial matrix.

It completes the breakdown of glucose by oxidizing a derivative of pyruvate to carbon dioxide.
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
Several steps in glycolysis and the citric acid cycle are redox reactions in which dehydrogenase
enzymes transfer electrons from substrates to NAD+, forming NADH.

NADH passes these electrons to the electron transport chain.

In the electron transport chain, the electrons move from molecule to molecule until they combine with
molecular oxygen and hydrogen ions to form water.

As they are passed along the chain, the energy carried by these electrons is transformed in the
mitochondrion into a form that can be used to synthesize ATP via oxidative phosphorylation.

The inner membrane of the mitochondrion is the site of electron transport and chemiosmosis, processes
that together constitute oxidative phosphorylation.


Oxidative phosphorylation produces almost 90% of the ATP generated by respiration.
Some ATP is also formed directly during glycolysis and the citric acid cycle by substrate-level
phosphorylation.

Here an enzyme transfers a phosphate group from an organic substrate to ADP, forming ATP.

For each molecule of glucose degraded to carbon dioxide and water by respiration, the cell makes up to
38 ATP, each with 7.3 kcal/mol of free energy.

Respiration uses the small steps in the respiratory pathway to break the large denomination of energy
contained in glucose into the small change of ATP.

The quantity of energy in ATP is more appropriate for the level of work required in the cell.
Glycolysis harvests chemical energy by oxidizing glucose to pyruvate

During glycolysis, glucose, a six carbon-sugar, is split into two three-carbon sugars.

These smaller sugars are oxidized and rearranged to form two molecules of pyruvate, the ionized form
of pyruvic acid.

Each of the ten steps in glycolysis is catalyzed by a specific enzyme.

These steps can be divided into two phases: an energy investment phase and an energy payoff phase.

In the energy investment phase, the cell invests ATP to provide activation energy by phosphorylating
glucose.

This requires 2 ATP per glucose.

In the energy payoff phase, ATP is produced by substrate-level phosphorylation and NAD+ is reduced
to NADH by electrons released by the oxidation of glucose.

The net yield from glycolysis is 2 ATP and 2 NADH per glucose.


No CO2 is produced during glycolysis.
Glycolysis can occur whether O2 is present or not.
The citric acid cycle completes the energy-yielding oxidation of organic molecules

More than three-quarters of the original energy in glucose is still present in the two molecules of
pyruvate.

If oxygen is present, pyruvate enters the mitochondrion where enzymes of the citric acid cycle
complete the oxidation of the organic fuel to carbon dioxide.

After pyruvate enters the mitochondrion via active transport, it is converted to a compound called
acetyl coenzyme A or acetyl CoA.

This step is accomplished by a multienzyme complex that catalyzes three reactions:
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1.
A carboxyl group is removed as CO2.
2.
The remaining two-carbon fragment is oxidized to form acetate. An enzyme transfers the pair of
electrons to NAD+ to form NADH.
3.
Acetate combines with coenzyme A to form the very reactive molecule acetyl CoA.

Acetyl CoA is now ready to feed its acetyl group into the citric acid cycle for further oxidation.

The citric acid cycle is also called the Krebs cycle in honor of Hans Krebs, who was largely
responsible for elucidating its pathways in the 1930s.

The citric acid cycle oxidizes organic fuel derived from pyruvate.


The citric acid cycle has eight steps, each catalyzed by a specific enzyme.

The acetyl group of acetyl CoA joins the cycle by combining with the compound oxaloacetate,
forming citrate.

The next seven steps decompose the citrate back to oxaloacetate. It is the regeneration of
oxaloacetate that makes this process a cycle.

Three CO2 molecules are released, including the one released during the conversion of pyruvate to
acetyl CoA.
The cycle generates one ATP per turn by substrate-level phosphorylation.

A GTP molecule is formed by substrate-level phosphorylation.

The GTP is then used to synthesize an ATP, the only ATP generated directly by the citric acid
cycle.

Most of the chemical energy is transferred to NAD + and FAD during the redox reactions.

The reduced coenzymes NADH and FADH2 then transfer high-energy electrons to the electron
transport chain.

Each cycle produces one ATP by substrate-level phosphorylation, three NADH, and one FADH2 per
acetyl CoA.
During oxidative phosphorylation, chemiosmosis couples electron transport to ATP synthesis
The inner mitochondrial membrane couples electron transport to ATP synthesis.

Only 4 of 38 ATP ultimately produced by respiration of glucose are produced by substrate-level
phosphorylation.


NADH and FADH2 account for the vast majority of the energy extracted from the food.


Two are produced during glycolysis, and 2 are produced during the citric acid cycle.
These reduced coenzymes link glycolysis and the citric acid cycle to oxidative phosphorylation,
which uses energy released by the electron transport chain to power ATP synthesis.
The electron transport chain is a collection of molecules embedded in the cristae, the folded inner
membrane of the mitochondrion.

The folding of the cristae increases its surface area, providing space for thousands of copies of the
chain in each mitochondrion.

Most components of the chain are proteins bound to prosthetic groups, nonprotein components
essential for catalysis.

Electrons drop in free energy as they pass down the electron transport chain.

During electron transport along the chain, electron carriers alternate between reduced and oxidized
states as they accept and donate electrons.
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
Each component of the chain becomes reduced when it accepts electrons from its “uphill”
neighbor, which is less electronegative.

It then returns to its oxidized form as it passes electrons to its more electronegative “downhill”
neighbor.

Electrons carried by NADH are transferred to the first molecule in the electron transport chain, a
flavoprotein.

The electrons continue along the chain that includes several cytochrome proteins and one lipid carrier.



The prosthetic group of each cytochrome is a heme group with an iron atom that accepts and
donates electrons.
The last cytochrome of the chain, cyt a3, passes its electrons to oxygen, which is very electronegative.

Each oxygen atom also picks up a pair of hydrogen ions from the aqueous solution to form water.

For every two electron carriers (four electrons), one O 2 molecule is reduced to two molecules of
water.
The electrons carried by FADH2 have lower free energy and are added at a lower energy level than
those carried by NADH.

The electron transport chain provides about one-third less energy for ATP synthesis when the
electron donor is FADH2 rather than NADH.

The electron transport chain generates no ATP directly.

Its function is to break the large free energy drop from food to oxygen into a series of smaller steps that
release energy in manageable amounts.

How does the mitochondrion couple electron transport and energy release to ATP synthesis?

The answer is a mechanism called chemiosmosis.

A protein complex, ATP synthase, in the cristae actually makes ATP from ADP and P i.

ATP uses the energy of an existing proton gradient to power ATP synthesis.

The proton gradient develops between the intermembrane space and the matrix.

The proton gradient is produced by the movement of electrons along the electron transport chain.

The chain is an energy converter that uses the exergonic flow of electrons to pump H + from the matrix
into the intermembrane space.

The protons pass back to the matrix through a channel in ATP synthase, using the exergonic flow of H +
to drive the phosphorylation of ADP.

Thus, the energy stored in a H+ gradient across a membrane couples the redox reactions of the electron
transport chain to ATP synthesis.

From studying the structure of ATP synthase, scientists have learned how the flow of H + through this
large enzyme powers ATP generation.

ATP synthase is a multisubunit complex with four main parts, each made up of multiple polypeptides:

1.
A rotor in the inner mitochondrial membrane.
2.
A knob that protrudes into the mitochondrial matrix.
3.
An internal rod extending from the rotor into the knob.
4.
A stator, anchored next to the rotor, which holds the knob stationary.
Protons flow down a narrow space between the stator and rotor, causing the rotor and its attached rod
to rotate.
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




The spinning rod causes conformational changes in the stationary knob, activating three catalytic
sites in the knob where ADP and inorganic phosphate combine to make ATP.
How does the inner mitochondrial membrane generate and maintain the H + gradient that drives ATP
synthesis in the ATP synthase protein complex?

Creating the H+ gradient is the function of the electron transport chain.

The ETC is an energy converter that uses the exergonic flow of electrons to pump H + across the
membrane from the mitochondrial matrix to the intermembrane space.
The ATP synthase molecules are the only place that H+ can diffuse back to the matrix.

The exergonic flow of H+ is used by the enzyme to generate ATP.

This coupling of the redox reactions of the electron transport chain to ATP synthesis is called
chemiosmosis.
How does the electron transport chain pump protons?

Certain members of the electron transport chain accept and release H + along with electrons.

At certain steps along the chain, electron transfers cause H + to be taken up and released into the
surrounding solution.
The electron carriers are spatially arranged in the membrane in such a way that protons are accepted
from the mitochondrial matrix and deposited in the intermembrane space.

The H+ gradient that results is the proton-motive force.

The gradient has the capacity to do work.

Chemiosmosis is an energy-coupling mechanism that uses energy stored in the form of an H + gradient
across a membrane to drive cellular work.

In mitochondria, the energy for proton gradient formation comes from exergonic redox reactions, and
ATP synthesis is the work performed.

Chemiosmosis in chloroplasts also generates ATP, but light drives the electron flow down an electron
transport chain and H+ gradient formation.
Here is an accounting of ATP production by cellular respiration.

During cellular respiration, most energy flows from glucose  NADH  electron transport chain 
proton-motive force  ATP.

Let’s consider the products generated when cellular respiration oxidizes a molecule of glucose to six
CO2 molecules.

Four ATP molecules are produced by substrate-level phosphorylation during glycolysis and the citric
acid cycle.

Many more ATP molecules are generated by oxidative phosphorylation.

Each NADH from the citric acid cycle and the conversion of pyruvate contributes enough energy to the
proton-motive force to generate a maximum of 3 ATP.

The NADH from glycolysis may also yield 3 ATP.

Each FADH2 from the citric acid cycle can be used to generate about 2 ATP.

Cellular respiration is remarkably efficient in energy conversion.
Feedback mechanisms control cellular respiration.

Basic principles of supply and demand regulate the metabolic economy.
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

If a cell has an excess of a certain amino acid, it typically uses feedback inhibition to prevent the
diversion of intermediary molecules from the citric acid cycle to the synthesis pathway of that
amino acid.
The rate of catabolism is also regulated, typically by the level of ATP in the cell.

If ATP levels drop, catabolism speeds up to produce more ATP.

Control of catabolism is based mainly on regulating the activity of enzymes at strategic points in the
catabolic pathway.

One strategic point occurs in the third step of glycolysis, catalyzed by phosphofructokinase.


This enzyme catalyzes the earliest step that irreversibly commits the substrate to glycolysis.

Phosphofructokinase is enzyme with receptor sites for specific inhibitors and activators.

It is inhibited by ATP and stimulated by AMP (derived from ADP).

When ATP levels are high, inhibition of this enzyme slows glycolysis.

As ATP levels drop and ADP and AMP levels rise, the enzyme becomes active again and
glycolysis speeds up.
Citrate, the first product of the citric acid cycle, is also an inhibitor of phosphofructokinase.

This synchronizes the rate of glycolysis and the citric acid cycle.

If intermediaries from the citric acid cycle are diverted to other uses (e.g., amino acid synthesis),
glycolysis speeds up to replace these molecules.

Metabolic balance is augmented by the control of other enzymes at other key locations in glycolysis
and the citric acid cycle.

Cells are thrifty, expedient, and responsive in their metabolism.
Glycolysis and the citric acid cycle connect to many other metabolic pathways

Glycolysis can accept a wide range of carbohydrates for catabolism.

Polysaccharides like starch or glycogen can be hydrolyzed to glucose monomers that enter
glycolysis.

Other hexose sugars, such as galactose and fructose, can also be modified to undergo glycolysis.

The other two major fuels, proteins and fats, can also enter the respiratory pathways used by
carbohydrates.

Proteins must first be digested to individual amino acids.

Amino acids that will be catabolized must have their amino groups removed via deamination.

The nitrogenous waste is excreted as ammonia, urea, or another waste product.

The carbon skeletons are modified by enzymes and enter as intermediaries into glycolysis or the citric
acid cycle, depending on their structure.

Catabolism can also harvest energy stored in fats.

Fats must be digested to glycerol and fatty acids.


Glycerol can be converted to glyceraldehyde phosphate, an intermediate of glycolysis.

The rich energy of fatty acids is accessed as fatty acids are split into two-carbon fragments via
beta oxidation.

These molecules enter the citric acid cycle as acetyl CoA.
A gram of fat oxides by respiration generates twice as much ATP as a gram of carbohydrate.
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
The metabolic pathways of respiration also play a role in anabolic pathways of the cell.

Intermediaries in glycolysis and the citric acid cycle can be diverted to anabolic pathways.


For example, a human cell can synthesize about half the 20 different amino acids by modifying
compounds from the citric acid cycle.

Glucose can be synthesized from pyruvate; fatty acids can be synthesized from acetyl CoA.
Glycolysis and the citric acid cycle function as metabolic interchanges that enable cells to convert one
kind of molecule to another as needed.


For example, excess carbohydrates and proteins can be converted to fats through intermediaries of
glycolysis and the citric acid cycle.
Metabolism is remarkably versatile and adaptable.
Fermentation enables some cells to produce ATP without the use of oxygen

Without electronegative oxygen to pull electrons down the transport chain, oxidative phosphorylation
ceases.

However, fermentation provides a mechanism by which some cells can oxidize organic fuel and
generate ATP without the use of oxygen.

In glycolysis, glucose is oxidized to two pyruvate molecules with NAD + as the oxidizing agent.

Glycolysis is exergonic and produces 2 ATP (net).

If oxygen is present, additional ATP can be generated when NADH delivers its electrons to the
electron transport chain.

Glycolysis generates 2 ATP whether oxygen is present (aerobic) or not (anaerobic).

Anaerobic catabolism of sugars can occur by fermentation.

Fermentation can generate ATP from glucose by substrate-level phosphorylation as long as there is a
supply of NAD+ to accept electrons.

If the NAD+ pool is exhausted, glycolysis shuts down.

Under aerobic conditions, NADH transfers its electrons to the electron transfer chain, recycling
NAD+.

Under anaerobic conditions, various fermentation pathways generate ATP by glycolysis and recycle
NAD+ by transferring electrons from NADH to pyruvate or derivatives of pyruvate.

In alcohol fermentation, pyruvate is converted to ethanol in two steps.


First, pyruvate is converted to a two-carbon compound, acetaldehyde, by the removal of CO2.

Second, acetaldehyde is reduced by NADH to ethanol.

Alcohol fermentation by yeast is used in brewing and winemaking.
During lactic acid fermentation, pyruvate is reduced directly by NADH to form lactate (the ionized
form of lactic acid) without release of CO2.

Lactic acid fermentation by some fungi and bacteria is used to make cheese and yogurt.

Human muscle cells switch from aerobic respiration to lactic acid fermentation to generate ATP
when O2 is scarce.

The waste product, lactate, may cause muscle fatigue, but ultimately it is converted back to
pyruvate in the liver.
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